Non-crimp fabrics

ABSTRACT

A method of producing a non-crimp fabric ( 10 ) and a non-crimp fabric comprising a layup of fabric plies ( 12 ) stitched together by a thread ( 18 ), at least part of which is fusible, is provided. The method can include heating the stitched plies ( 12 ) to soften or melt the fusible thread ( 18 ). The use of fusible thread ( 18 ) can act as an in situ binder within the non-crimp fabric ( 10 ) and tension created by the unfused stitching may create channels for matrix resin infusion during manufacture.

[0001] The invention relates to non-crimp fabrics and is particularlyconcerned with non-crimp fabrics with improved drape characteristics.

[0002] It is known to produce fabrics, by laying several plies of fibresinto a stack of required thickness. The fabrics may be of a non-crimpedor warp-knitted type. The fabrics maybe unidirectional (fibres in asingle orientation) or more typically multi-axial. The term“multi-axial” means that alternate plies of fibres will be constructedin several different directions to produce a fabric with optimumstrength and stiffness in required directions. Commonly used directionsare 0°, +45°, −45° and 90°. Normally, and for the purposes of thisinvention, the 0° plies are in the warp direction and the 90° plies inthe weft direction, but other layers may contain fibres at differentangles, typically at 45° or −45°. The term “Non-crimp” refers to fabricswhere one or multiple layers of fibres are laid upon each other andtransformed into a fabric by stitching or application of a binder suchthat the fibres remain straight and without substantial crimp. As wellas offering performance advantages relative to traditional wovenmaterials, non-crimp fabrics (NCFs) may be cheaper to produce per unitmass and be faster to manufacture. Such composites have potential foruse in the production of wing skins for example.

[0003] However, as the number of layers in a non-crimped fabric blanketincreases, the ability to conform to changes in shape-of the componentbeing made (drape) decreases due to the restrictions imposed by thestitching. Consequently, the drape characteristics of unidirectional andbiaxial NCFs are generally comparable to that of woven fabrics whereasdrape is increasingly restricted with triaxials, quadraxials, etc. hencerestricting possible applications.

[0004] The present application is concerned with non-crimp fabricshaving improved drape properties.

[0005] The prime role of the stitching is to hold the plies of thenon-crimp fabric together during handling in a predominantly un-crimpedmanner. However, if the density of stitches is too high or the stitchingis under too much tension or too thick then disruption and crimping ofthe fibres may occur.

[0006] It is also known to use a binder powder for pre-form manufactureusing non-crimp fabrics. However, the use of such fine powders may behazardous as the powder may inhaled or irritate the skin of anoperative.

[0007] According to the invention there is provided a method ofproducing a non-crimp fabric which comprises providing a laminate ofplies and stitching the plies together using a fusible thread.

[0008] In use, the primary function of the stitch is to hold the fibrestogether during the non-crimp fabric manufacture and to aid resininfusion. Whilst the stitch initially restricts drape, the eventualfusion of the stitch overcomes that restriction. In the presentapplication, the term fusible means that the thread not only melts byitself when heated, but may also dissolve into a resin during processingat temperature. Therefore, by using a fusible thread, the stitched pliescan be heated so as to soften or melt the fusible thread so as toimprove the drape of the fabric.

[0009] Preferably, the plies are warp and weft dominated. Examples ofwarp and weft dominated plies are described in EP-A-0826438 to which thereader is directed for details. Fibres of each ply used in the compositemay be constructed in several different directions. In the presentapplication they are preferably 0°, +45°, −45° and 90°, but angles of±20° to 90° can be used on the weft orientation.

[0010] The non-crimp fabric may comprise from 0.5% to 10% fusible threadby mass. More preferably, the non-crimp fabric comprises from 1% to 3%fusible thread by mass.

[0011] The fusible thread will preferably melt at a temperature ofbetween 40° C. to 150° C., depending on application and processing routeused. The fused thread may react with the curing resin or remain as aseparate phase, preferably causing minimal decrease in mechanicalproperties and improving damage tolerance. Preferably, after curing, thefusible thread should have a glass transition temperature (T_(g)) ofgreater than 120° C. for aerospace applications. To balance the meltingand T_(g) temperature, some reaction between the fused stitch and resinis desirable. A T_(g) greater than 120° C. would improve hot-wetperformance, especially in compression. A T_(g) of less than 120° C. mayhelp damage tolerance.

[0012] The fusible thread may be constructed from one or more differentmaterials. Where a plurality of materials is used, the threads may fuseat different temperatures or contain both fusible and infusiblematerials. If one material is infusible, the fibres are preferably notcontinuous, thus allowing movement when the fusible material melts.Infusible thread materials at normal processing temperatures include:polyester, acrylamide polymers, e.g. Kevlar®, carbon, acrylonitrilepolymers e.g. Panox®, or PBO. Fusible thread materials may includepolyamides, lower molecular weight polymers of polyethersulphone e.g.Grillon K-110 from EMS Chemie AG or blends of epoxy resins.

[0013] The fusible thread may be constructed from two or more differentmaterials, each having a different fusion temperature.

[0014] Preferably, the stitching runs substantially transversely throughthe plies and follows a predetermined pattern. The pattern may be tricotclosed, open pillar stitch, closed pillar stitch, open tricot-pillarstitch, or closed tricot-pillar stitch or variants thereof.

[0015] The method may also include heating the plies as aforesaid toimprove the drape of the fabric. In one embodiment, the plies are heatedto a temperature not greater than the fusion temperature of the thread.However, in another embodiment, the plies are heated to a temperature ator above the fusion temperature of the fusible thread, so that thethread fuses and allows the fabric to move. Where this occurs withoutthe presence of a matrix resin, cooling under pressure below the fusiontemperature substantially maintains the shape formed creating apre-form, due to the fusible stitch acting as an in-situ binder.

[0016] Tension in the un-fused stitching tends to create channels in thenon-crimp fabric which are normally undesirable. However, it has beenfound that the method may facilitate easier matrix resin infusion intothe fabric through the channels, and then heating the fabric to atemperature above the fusion temperature of the stitching whereby thestitching fuses and relieves the tension. In that way fibres in thenon-crimp fabric, which have been crimped due to tension in thestitching, will move to minimise crimp, reduce resin rich areas andimprove ultrasonic NDT inspection.

[0017] Where the method includes infusing the non-crimp fabric with amatrix resin, the fusible thread will preferably be chemicallycompatible with the matrix resin and be of a similar modulus.

[0018] The fusible thread may melt and remain as a separate phase fromthe matrix resin, dissolve in the matrix or precipitate out later as thematrix is cured or allowed to set.

[0019] The matrix may be a resin.

[0020] The fused stitching may also act as a toughening phase withinresin infused non-crimp fabric. Such a secondary benefit would benormally at fusible stitch levels of 5-10% of fabric mass

[0021] A hairy yarn as disclosed in applicants co-pending UK PatentApplication filed simultaneously herewith and entitled ‘Composites’ maybe introduced into the plies so as to improve the shear propertiesbetween the plies and improve damage tolerance.

[0022] A non-crimp fabric in accordance with the invention will now bedescribed by way of example and with reference to the accompanyingdrawings in which:

[0023]FIG. 1 is a schematic cutaway view of a non-crimp fabric beingstitched with a fusible thread; and

[0024]FIG. 2 is a table showing different types of stitch patterns.

[0025] Referring to FIG. 1, a non-crimp fabric 10 is constructed from alaminate 12 of plies 14 a-14 e. The plies are multi-axial with fibres inrespective plies 14 a to 14 e at angles of 0°, 90°, +45°, 90° and −45°as shown in FIG. 1.

[0026] A set of needles 16 is used to facilitate the stitching of theplies 12 together. The needles 16 move transversely up and down throughthe laminate 12 in a predetermined pattern whilst the laminate is movedin a direction shown by the arrow ‘A’. Examples of predetermined stitchpatterns used can be seen in FIG. 2.

[0027] A fusible thread 18, for example made of Grillon K-110®, isthreaded into the needles 16. The thread 18 is then stitched into thelaminate 12 in the predetermined pattern.

[0028] The non-crimp fabric 10 can then be used to produce structuralcomponents. The fabric 10 is draped over a pre-form tool to be copied(not shown), with the fabric being cut to shape first. The fabric 10 maybe heated to a temperature at or in excess of the fusion temperature ofthe thread so as to relieve some of the tension within the threadthereby improving the drape of the fabric.

[0029] In another embodiment of the invention, a resin (not shown) isinfused in to the non-crimped fabric 10. During the infusion process theresin passes through channels (not shown) resulting from tension in thestitching. After infusion, the fabric 10 is heated to a temperatureabove the fusion temperature of the thread. This causes the stitching tofuse and thereby relieve the tension. As a result, fibres in thenon-crimp fabric 10, which have been crimped due to tension in thestitching, can move to minimise crimp.

[0030] In still another embodiment of the invention a ‘hairy yarn’ (notshown) may be used. Strands of hairy yarn are substituted for fibreswithin the plies 14. The strands of hairy yarn in one ply interact withthose in another thereby improving the shear strength properties of thenon-crimp fabric 10 as described in the aforementioned UK PatentApplication to which the reader is directed for full details.

1. A method of draping a non crimp fabric over a forming tool, themethod including the steps of: providing a non crimp fabric comprising alayup of plies stitched together using a thread at least part of whichis fusible; placing the non crimp fabric on the forming tool, andheating the non crimp fabric sufficiently at least to soften the fusiblethread as the non crimp fibric drapes over the tool, whereby to reducethe restriction to drape caused by the stitching.
 2. A method accordingto claim 1 in which the plies are heated to a temperature not greaterthan the fusion temperature of the thread.
 3. A method according toclaim 1 in which the plies are heated to a temperature above the fusiontemperature of the thread.
 4. A method according to claim 3 in whichpressure is applied to conform the plies to the shape of the tool andthe plies are cooled, while under pressure, whereby to create a preformwith the fused thread acting as an in situ binder within the non-crimpfabric.
 5. A method according to any one of claims 1 to 4 includingcreating tension in the un-fused stitching to create resin transferchannels in the non-crimp fabric.
 6. A method according to any precedingclaim in which matrix resin is infused through the non-crimp fabric. 7.A method according to claims 5 or 6 including selecting a matrix resinwhich is chemically compatible with the fusible thread.
 8. A methodaccording to any one of claims 5 to 7 including selecting a matrix resinand a fusible thread with a similar modulus.
 9. A method according toany one of claims 5 to 8 in which the fusible thread remains as aseparate phase from the matrix resin, dissolves in the matrix resin orprecipitates out as the matrix resin is cured or allowed to set.
 10. Amethod according to any one of claims 5 to 9 in which the stitching isselected so that the fused stitching acts as a toughening phase withinthe resin infused non-crimp fabric.
 11. A method according to anypreceding claim in which the non crimp fabric provided comprises from0.5% to 10% fusible thread by mass.
 12. A method according to claim 11in which the non-crimp fabric provided comprises from 1% to 3% fusiblethread by mass.
 13. A method according to any preceding claim includingmelting or dissolving the fusible thread at a temperature between 40° C.and 250° C.
 14. A method according to any preceding claim includingselecting a fusible thread with a glass transition temperature (T_(g))of greater than 120° C. after curing.
 15. A method according to anypreceding claim in which the fusible thread is constructed from two ormore parts comprising different materials, each having a differentfusion temperature.
 16. A method according to claim 15 in which at leastone of the materials from which the thread is constructed is infusible.17. A method according to claim 16 in which the said infusible part ofthe thread is non-continuous.
 18. A method according to claim 16 or 17in which the said infusible part of the thread is of polyester.
 19. Amethod according to any preceding claim in which the stitching is runsubstantially transversely through the plies.
 20. A method according toclaim 19 in which the stitching is carried out in a predeterminedpattern and is selected from the group: tricot closed, open pillarstitch, closed pillar stitch, open tricot-pillar stitch or closedtricot-pillar stitch.
 21. A method according to any preceding claimincluding providing a said non crimp fabric in which the plies aremulti-axial.
 22. A method according to any preceding claim in whichfibres of each ply are placed in directions of 0°, +45°, −45° and 90°.23. A method according to any preceding claim in which fibres of eachply are placed in directions of ±20° to 90°.
 24. A method according toany preceding claim in which a hairy yam is introduced into the plies.